The continued expansion of mobile computing applications requires the packing of ever growing levels of computing power in smaller device footprints. Designers of semiconductor devices are relying on the use of a variety of new chip architectures to meet the new device requirements. These new architectures include flip chip wafer bumping using copper pillars as well as approaches employing through silicon vias (TSV) such as three-dimensional integrated circuits (3D IC) in which wafers are thinned, the resulting dies are stacked and then connected by TSV and 2.5D interposer designs. These approaches pose significant challenges not only for the designers of these new IC architectures, but for the designers of the packaging materials that will be used for these devices.
Material requirements for packaging applications are continuously evolving as these new, advanced devices are relying heavily on wafer level packaging (WLP) and 3D integration. While there are a number of traditional packaging materials that have been employed through the years, polyimides, due to their excellent electrical, mechanical and thermal properties, have been the material of choice for semiconductor packaging applications. However, drawbacks of conventional polyimides include high cure temperatures (>350° C.), high post-cure shrinkage and high levels of moisture absorption. This high shrinkage leads to cured polyimide films having high residual stress which leads to bowing of the silicon wafer. The next generation chip architectures employing 3D integration require that the silicon wafers be thinned down up to 20 μm in most advanced applications in order to satisfy requirements for vertical integration. These thinned wafers are extremely fragile and excessive residual stress in the packaging material used could be catastrophic. The next generation packaging materials should be designed so as to impart minimal stress on the wafer. For this reason, low cure temperature and low post-cure shrinkage are among the important requirements for advanced packaging materials.
Soluble polyimide polymers are known and the photosensitive compositions from those polymers have been reported. In general, when the molecular weight of the polyimide polymers are low, acceptable lithographic performance is observed. However, the mechanical properties are degraded to such an extent that compositions employing those polymers cannot be used as advanced packaging materials in the packaging process. When the polyimide molecular weight is increased, acceptable mechanical properties are achieved. However, the solubility of the resulting polyimide compositions in a developer decreases and poor lithographic performance is obtained. This disclosure describes polyimide compositions yielding both good mechanical properties and good lithographic performance suitable for advanced packaging applications.